<!DOCTYPE html>
<html lang="en">
<head>
  <meta charset="UTF-8">
  <meta name="viewport" content="width=device-width, initial-scale=1.0">
  <title>盒子模型</title>
<style>
    div {
    width: 400px;
    height: 400px;
    border-width: 5px;
    border-style: dotted;
    /* border-width 边框的粗细 一般情况下都用px */
    border-color: pink;

  }
</style>
</head>
<body>
  <div>
  这是一个有边框的盒子
  </div>
</body>
</html>